Problem: Michael is 3 times as old as Christopher and is also 20 years older than Christopher. How old is Christopher?
Answer: We can use the given information to write down two equations that describe the ages of Michael and Christopher. Let Michael's current age be $m$ and Christopher's current age be $c$ $m = 3c$ $m = c + 20$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $c$ , and both of our equations have $m$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3c$ $-$ $ (c + 20)$ which combines the information about $c$ from both of our original equations. Solving for $c$ , we get: $2 c = 20$ $c = 10$.